(1) Field of the Invention
The present invention relates to the field of sensors for measuring physical properties of a web, and more particularly to a sensor and method for measuring moisture in a web by detecting the amount of infrared radiation in various wavelength regions after being transmitted through or reflected from the web. The measurement of the moisture content is optimally insensitive to the temperature, the basis weight and the composition of the web over a wide range of moisture, temperature and basis weights.
(2) Description of the Related Art
Paper is produced in a moving sheet called a "web." Because a paper web is produced from an aqueous suspension including wood pulp fibers, cotton fibers and various chemicals, the web initially contains a considerable amount of moisture. Most of this moisture is removed during the papermaking process. However, for a variety of reasons, it is desirable to maintain at least some moisture in the web. For example, if the web is too dry, it will tend to curl at the edges.
A paper web is normally dried by passing it around heated steel drying drums. However, this technique tends to dry the web unevenly. Such drying produces paper of uneven quality. Thus, various devices have been developed to moisten or dry only portions of the web. Typically, the moistening or drying occurs after the web has passed around the drying drums, to produce a web having a uniform moisture content. Of course, the paper mill operator, or the paper mill's process control computer, must know the moisture profile of the web before these moistening and drying devices can be used effectively Web moisture sensors have therefore been developed which scan back and forth across the cross-directional width of the web to determine its moisture content at various locations.
Water absorbs infrared radiation. Certain types of web moisture sensors take advantage of this phenomenon by directing a beam of infrared radiation at a web and measuring the intensity of the infrared beam after it passes through the web. The more moisture in a web, the greater the absorption of the infrared radiation in specific wavelength regions.
Some of these infrared moisture sensors use two infrared detectors with an infrared band pass filter positioned in front of each detector. The pass band of each filter is chosen so that each detector receives radiation in a narrow wavelength region of the infrared spectrum. One filter is chosen to pass infrared radiation in a region of strong absorption by the water in the web. Thus, the detector associated with this filter is sensitive primarily to the amount of water in the web. This first detector receives more infrared radiation when the web is dry and less infrared radiation when the web is moist.
A second band pass filter associated with a second detector is selected to pass infrared radiation in a region of the infrared spectrum where there is relatively little absorption by moisture. In this region, most of the absorption is due to the web fibers themselves, not to the moisture in the web. Thus, as the web weight per unit area (i.e., the "basis weight") increases, this second detector receives less infrared radiation. The output of this second detector is used to compensate the moisture measurements of the first detector for changes in the basis weight of the web. When the outputs from these two detectors are properly combined, these types of moisture sensors provide a measurement of the amount of moisture contained in the web or the percentage of moisture in the web, so that the moisture measurement is not affected by changes in the basis weight of the web.
However, the intensity of the transmitted infrared beam is not only dependent upon the moisture content and basis weight of the web. The absorption of infrared radiation by the moist web also varies with wavelength. The water and web fibers absorb certain wavelengths of the infrared spectrum more effectively than other wavelengths so that there are absorption peaks and valleys at various wavelengths along the spectrum. Moreover, these peaks and valleys shift to shorter wavelengths with increases in web temperature and to longer wavelengths with decreases in web temperature.
However, the previously described infrared moisture measuring devices fail to compensate for shifts in the infrared absorption spectrum resulting from changes in web temperature. Because on-line paper web temperatures may range from 10.degree. C. to as high as 100.degree. C., the moisture measurements of these devices are subject to significant error.
U.S. Pat. No. 4,928,013 to Howarth et al. ("Howarth") describes an infrared moisture sensor with two band pass filters which are selected to compensate for web temperature change. In this sensor, a first band pass filter, associated with a measure detector, is selected so that it is approximately centered around the infrared absorption peak for water, at about 1.93 microns. As the web temperature increases, the intensity of detected infrared radiation increases at the long wavelength side of the pass band filter, while an approximately equal decrease in the detected infrared occurs at the opposite short wavelength side of the pass band. With this technique, the total amount of infrared radiation reaching the measure detector is substantially insensitive to web temperature. A second band pass filter, associated with a reference detector, is selected so that it is in a region of the infrared spectrum which is predominantly absorbed by the web fibers The intensity of the infrared beam detected by the reference detector is primarily indicative of the basis weight of the web and is used to compensate for changes in basis weight.
The moisture sensor described in Howarth has the temperature insensitivity desired for the moisture measurements of some light weight paper, but may not achieve the degree of accuracy required by all customers, and more importantly, cannot achieve, the desired accuracy for heavier grades of paper sometimes encountered in paper manufacturing.
When band pass filters are manufactured there are inevitable variations in the exact band pass center wavelength and width, and for broader filters, in the shape of the transmission envelope. Thus, even with the lighter grades of paper, there is still some temperature sensitivity in a two band pass filter sensor which cannot be removed and may affect the desired accuracy of the moisture measurements.
The measurement of the moisture content of heavier grades of paper with such a sensor has additional problems. Each grade of paper has its own characteristic transmission spectra. For example, paper usually contains between 2-12% moisture by weight. Thus, a heavier grade of paper will typically contain more moisture. However, as the amount of moisture increases, the water absorption peak increases in magnitude as well as broadens in the wavelength direction. Both of these effects tend to reduce the amount of radiation transmitted through the web. In fact, at the water absorption peak, the strong water and cellulose absorptions of the heavier grades of paper may effectively absorb nearly all of the radiation directed at the web from the infrared source. A narrow set of band pass filters which may be adequate for a light weight paper may be entirely inadequate in terms of transmitting the required amount of light through a heavier grade of paper.
Moreover, a moisture sensor with a set of filters which are nearly temperature insensitive for a lighter grade of paper may become very temperature sensitive as the shape of the transmission spectra changes with a heavier grade of paper, especially if the water absorption peak shifts partly or entirely outside the measurement region. Finally, even if the measure and reference filters are carefully selected there is still some residual temperature sensitivity when measuring heavier grades of paper which cannot be removed by a moisture sensor using a two band pass filter system.
Howarth also describes an infrared moisture sensor with a detecting unit which measures the transmission of the infrared beam through the web at three separate wavelength regions of the spectrum. The first band pass filter of the three filters passes infrared radiation having wavelengths within a slope of the absorption spectrum adjacent to the water absorption peak. Infrared radiation within this region is readily absorbed by water, but provides a stronger signal than if the pass band of the filter were centered directly over the water absorption peak, at about 1.93 microns. However, as a result, the output of the measure detector associated with this first filter becomes temperature sensitive.
A second band pass filter is selected to pass wavelengths in a region of the spectrum absorbed primarily by paper fibers. Therefore, the intensity of the infrared beam detected by the second detector is primarily indicative of the basis weight of the web. The output of the second detector is therefore used to correct the moisture measurement of the first detector for changes in basis weight.
A third band pass filter is selected to compensate for the temperature sensitivity of the first detector. The third filter passes a region of the spectrum where the water absorption of the infrared radiation varies rapidly with wavelength. For example, the third filter is preferably located on a slope of the absorption spectrum between a peak and a valley. Since the infrared spectrum for both water and paper fiber shifts to lower wavelengths with increases in web temperature, a third detector associated with the third filter is sensitive to web temperature. Since the third filter is located on a different portion of the infrared spectrum than the first filter, the output of the third detector will have a different temperature coefficient than the output of the first detector. Thus, the output of the third detector can be used to correct the moisture measurement of the first detector for the effects of changing web temperature.
However, the degree of compensation required by this technique to compensate for the temperature sensitivity of the measurement detector generates problems in certain contexts. For example, the third filter is typically selected to pass a region of the spectrum which is sensitive to not only the web temperature, but also to the composition and/or basis weight of the web. When the first and second filter are not selected to remove as much temperature sensitivity as possible, the high degree of temperature compensation required from the third detector may introduce significant dependence on the basis weight, ash, coating or pigment of the web. This dependence is normally removed by providing different calibration constants for different paper types.
A high degree of temperature compensation from the third detector introduces other sources of error. Howarth provides that the moisture of the web be calculated by combining the outputs from the three detectors in empirically derived equations. These equations combine values indicative of the amplitudes of each of the three detector signals to achieve a weighted average of these three values. The weighting coefficients of the equations, for example, C1 and C2, are dependent upon the width of the pass bands of each filter on the absorption spectrum. The coefficient values are chosen empirically to provide the weighted average of the detector signal values to minimize temperature and basis weight sensitivity.
The values for C1 and C2 are obtained by taking the readings of the three detector signals using samples of moisture-containing paper which are sealed between two plates of glass. To accomplish this, the glass-enclosed web is first heated. Then, measurements of the infrared penetration through the glass-enclosed web are made periodically as the temperature of the glass-enclosed web decreases.
In some applications, the amount of compensation needed for temperature insensitivity in glass-enclosed sample tests is different from that needed for on-line measurement. It would be highly desirable if the calibration constants obtained on samples at temperature around 22.degree. C. were correct for on-line measurements, even for paper in a very strongly non-equilibrium state at around 100.degree. C. As a practical matter, however, this method of calibrating the temperature compensation by glass-enclosed samples provides different values for the constant C1 than that required for on-line measurements. This is presumably due to the non-equilibrium condition of the paper on-line at a high temperature having a different affect on the absorption, for example, at 1.6 microns than on the absorption at 1.9 microns. This different value for the calibration constant can introduces an additional source of error in the moisture measurements. The high degree of compensation required exacerbates this source of error.
Thus, there is a need for a moisture sensor which can overcome these types of problems and accurately determine the moisture content of the web in a manner which is optimally insensitive to the temperature, the basis weight and the composition of the web over a wide range of moisture, temperature and basis weights.